The explosive growth of digital communications technology has resulted in an ever-increasing demand for bandwidth for communicating digital information, such as data, audio and/or video information. To keep pace with the increasing bandwidth demands, optical communication systems, with their large inherent channel capacities, frequently provide the backbone of modern communication systems. It is believed that the growth of fiber optic communications will continue for metropolitan and long-haul networks and that fiber optic communications will ultimately be applied even for access and local area (LAN) networks. In this on-going evolution of fiber optic communication, adaptive electronic equalization for combating impairments in fiber optic communication may play an important role in the following themes:
Adaptive electronic equalizers for impairment compensation in fiber optic networks have been frequently studied. Initially, the dominant noise was quantum, shot or electronic thermal noise, which can be modeled effectively as additive Gaussian noise. Since the advent of efficient and low-noise fiber amplifiers in 1987, optical amplifiers have been used extensively to increase the transmission distance without conversion between the optical and electrical domains. A number of studies have explored a variety of equalizer structures for adaptive optical channel impairment compensation (ranging from feed forward type equalizers to maximum-likelihood estimators). More recently, interest is more focused on adaptive polarization mode dispersion (PMD) compensation since, in the wide-spread deployment of 10 Gbps optical equipment, substantial unpredictable PMD is accumulated over a long distance of legacy fibers, enough to cause network outage, though polarization mode dispersion compensation still remains an open topic. A need therefore exists for improved techniques for compensating for polarization mode dispersion and other noise in optical receivers.